Particulate matter (PM) refers to a complex mixture of tiny solid particles and liquid droplets suspended in the air. These airborne particles vary widely in size and composition, originating from numerous sources. Scientists categorize them by aerodynamic diameter: PM10 includes particles 10 micrometers or less, and PM2.5 refers to fine particles 2.5 micrometers or less. While human activities contribute significantly to air pollution, natural processes also release these microscopic materials into the atmosphere.
Geological and Atmospheric Sources
Volcanic eruptions are a natural source of atmospheric particulate matter. These geological events eject large quantities of ash, including pulverized rock, minerals, and volcanic glass, into the atmosphere. Volcanoes also release gases like sulfur dioxide, which can react in the atmosphere to form sulfate aerosols. These sulfate particles can reflect sunlight, influencing regional climate patterns.
Wind erosion in arid and semi-arid regions generates plumes of mineral dust. Deserts such as the Sahara, Gobi, and Arabian are major global sources, where strong winds lift fine soil particles into the atmosphere. These dust storms can transport billions of tons of particulate matter over thousands of kilometers annually, significantly impacting air quality far from their origin. The composition of this dust includes silicates, oxides, and carbonates from the Earth’s crust.
Oceanic processes contribute to natural particulate matter through sea salt aerosol formation. When wind blows over the ocean surface, it generates waves and whitecaps, forming air bubbles. As these bubbles rise to the surface and burst, they eject droplets of seawater into the atmosphere. The water then evaporates, leaving behind salt crystals, primarily sodium chloride, which become airborne particles.
Biological and Combustion Sources
Wildfires, often ignited by lightning, release particulate matter into the atmosphere. The combustion of vegetation produces a mixture of smoke, soot (black carbon), and ash. Soot, a product of incomplete combustion, consists of fine carbonaceous particles and can absorb sunlight, impacting atmospheric radiation. These wildfire emissions can travel long distances, affecting air quality far from the burn site.
Pollen and spores are biological particulate matter, prevalent during specific seasons. Pollen grains, the reproductive cells of plants, are released into the air by trees, grasses, and weeds, especially during spring and summer. Fungal spores, reproductive units, are also widely dispersed by wind. These biological particles vary in size and shape but are within the PM10 category.
Airborne microorganisms contribute to natural particulate matter. Bacteria, viruses, and decaying organic material can become aerosolized from soil, vegetation, and water bodies. These biological aerosols are associated with dust or plant debris. While smaller in concentration compared to other natural sources, they represent a diverse group of biological particles.
Global Distribution and Transport
Once released into the atmosphere, particulate matter from natural sources can undergo long-range transport, traveling thousands of kilometers. Strong upper-level winds, such as the jet streams, act as high-altitude highways for these particles. The altitude at which particles are injected into the atmosphere influences their transport distance and duration. Higher injections allow for longer atmospheric residence times.
The Saharan dust plume is a well-documented example, frequently crossing the Atlantic Ocean to reach the Caribbean and the Americas. This transport can take several days to a week, carrying millions of tons of dust. Similarly, large wildfire events in Siberia or North America can generate smoke plumes that traverse entire continents, impacting air quality in distant regions. The movement of these plumes is governed by large-scale meteorological patterns.
Atmospheric circulation patterns dictate where these particles will deposit, either through dry deposition (settling out) or wet deposition (washed out by rain or snow). The persistence of these particles in the atmosphere depends on their size and atmospheric conditions. Larger particles tend to settle out more quickly, while finer particles can remain suspended for weeks, allowing for extensive global distribution.